1. Field of the Invention
The present invention relates generally to a magnetic data storage apparatus and method and in particular to a read channel for reading data from magnetic media, the read channel including equalization using a filter.
2. Description of the Related Art
Magnetic recording devices such as magnetic tape drives are used for recording computer data for storage and retrieval. Various techniques have been developed over the years to improve the characteristics of data reading and writing on the magnetic media.
On the read channel side, Finite Impulse Response (FIR) filters are widely used as a means of equalizing the read channel response to a given target response, for example PR4, EPR4, or the like. The response of these filters is controlled by a set of coefficients. Often the response of the FIR filter is changed during the operation of the tape drive in order to compensate for changes in channel characteristics, for example, changes in the recording media, the recording head, the electronics, and the like. Tape drives in particular must deal with the variations in the channel characteristics caused by the interchange of the recording media.
A general method for changing the FIR filter coefficients in order to optimize the FIR filter responses is by implementing LMS (Least Mean Square) hardware adaptability.
The LMS algorithm itself does not control the coefficients sufficiently well in an over-sampled tape drive read channel. In addition to the normal imperfections of the LMS algorithm caused by limited precision, the over-sampled channel also causes imperfections due to over sampling since there is no information from the detector about the over-sampled part of the frequency spectrum and because the detected bit position do no correspond exactly to the analog to digital sample positions.
Leaky LMS algorithms have been used to combat the issues of limited precision and coefficient drift. This is an extension of the general LMS algorithm as follows:Cx(n+1)=(1−d)Cxn+uSxnEn 
The previous coefficient is scaled down by a term (1−d) in the estimate of the new coefficient, where d is the leak down gain. This leakage limits coefficient magnitude drift, but does not control coefficient sideways drift.